Does your new Mac have a populated A3 library already? If not, then I would move the entire library over from your old Mac into your new Mac. A3 libraries can be moved intact.

If you have already put pictures into both Libraries... then you need to decide if you want to take the risk of duplicates (add). Merge will avoid exact duplicates... but could be a bit of a mess if they are organized very differently on the two libraries.

Finally, I have never just dragged projects across. Maybe it works, but I think the best way is: file>import>library. Just point it to your full existing library. You will still need to choose add or merge.

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